1. Field of the Invention
The present invention relates to an optical pulse compressor, and in particular to an optical pulse compressor used in a light source for generating high-intensity short optical pulses.
2. Description of the Related Art
One of the existing methods for generating high-intensity short optical pulses is a method of compressing optical pulses that are output from various optical pulse sources. There are roughly two kinds of pulse compressors based on the nonlinear effect of an optical fiber.
The first one is a pulse compressor that uses a normal dispersion fiber and a dispersion compensator.
In this case, the normal dispersion fiber has a role of giving a nearly linear positive chirp to an input pulse due to self-phase modulation (SPM) and group velocity dispersion (GVD) effects. An optical pulse, when input to an optical fiber having normal dispersion, is spectrally broadened and is positively chirped over the entire pulse width.
The pulse, after passing through the fiber, is sent to a dispersion compensator having a diffraction grating pair and is compressed due to anomalous GVD. Normal dispersion means that a delay on the time axis is larger for a shorter wavelength, and causing such dispersion is called positive chirping or giving a positive chirp. Anomalous dispersion means that the delay is larger for a longer wavelength. This compressor, using a normal dispersion fiber and a dispersion compensator, will be specifically called a chirp compensation compressor.
The second one is a pulse compressor that uses an anomalous fiber. In this case, an input pulse is compressed due to a soliton effect. A soliton is a phenomenon, or a pulse itself showing the phenomenon, in which the pulse propagates in a fiber without change in waveform because pulse broadening due to the GVD of the fiber balances pulse shortening due to the SPM caused by the anomalousness of the fiber.
Here, a high-intensity optical pulse is compressed during propagation through a fiber since the SPM effect increases with the intensity of the optical pulse. In this case, there is a tendency that the higher the intensity of the input pulse is, the higher the compression factor (input pulse width divided by output pulse width) is. This compressor with an anomalous dispersion fiber will be specifically called a high-order soliton compressor.
A paper by Tai et al. (Applied Phys. Lett., vol. 48, pp. 1034-1035 (1986)) shows that a high compression factor of 1100 is realized by a two-stage compressor that uses the above-described two methods in succession.
This experiment used a 100 ps (picoseconds) input pulse that was generated by a mode-locked neodymium:yttrium aluminum garnet (YAG) laser operating at a wavelength of 1.32 μm (micrometers). The first stage compression of the experiment produced a pulse compressed to a width of 2 ps using the above-described chirp compensation compressor with a normal dispersion fiber and a diffraction grating pair. This pulse was further compressed by the second stage high-order soliton compressor to obtain a resultant output pulse with a pulse width of 90 fs (femtoseconds).
The above described chirp compensation compressor has been favorably used for wavelengths shorter than the zero-dispersion wavelength (approximately 1.3 μm) of bulk quartz. This is because an ordinary quartz optical fiber shows normal dispersion for wavelengths shorter than the zero-dispersion wavelength of bulk quartz. On the other hand, an optical fiber can be realized that shows normal dispersion for wavelengths longer than the zero-dispersion wavelength (approximately 1.3 μm) of bulk quartz, by carefully choosing the material composing the fiber and the refractive index profile. Such an optical fiber can be realized by a fluoride fiber mainly composed of, for example, zirconium tetrafluoride (ZrF4) and hafnium tetrafluoride (HfF4).
By using an optical fiber that shows normal dispersion for wavelengths longer than the zero-dispersion wavelength of bulk quartz, a chirp compensation compressor can be applied to those wavelengths. The advantage of operation in those wavelengths includes realizing a low-cost and stable all-optical-fiber compressor by replacing a diffraction grating pair with a piece of anomalous optical fiber mainly composed of quartz.